Silicon steel is known for its excellent soft magnetic properties. Si steel essentially having an Si content of 3.5% by weight or less is usually employed as iron cores in power-frequency motors, transformers, etc. However, when such Si steel is used within a frequency range of 1 kHz or more, that is higher than commercial frequency, the iron loss caused by eddy currents is excessive. Therefore, Si steels of that type are disadvantageous for use in iron cores in many electric appliances.
With the recent tendency toward small-sized and high-performance electric appliances, there is an increasing demand for high-performance motors, high-frequency transformers, etc. They demand materials having small iron loss.
Within an extremely high frequency range (100 kHz or higher), the eddy-current loss in steel sheets is enormous. Therefore, for use in such an extremely high frequency range, ferrite has heretofore been employed as iron cores, even though its magnetic flux density is low.
In this connection, an increase of Si content of steel brings about an increase in its electric resistance, thereby resulting in reduction of the eddy currents induced in the steel. Therefore, the iron loss of such high-Si steel is favorably reduced within a frequency range higher than commercial frequency. However, Si steel having an Si content larger than 3.5% by weight is extremely hard and brittle, and its workability is poor. Therefore, it is extremely difficult to produce Si steel sheets of that type by rolling. In particular, the workability of Si steel having an Si content greater than 5.0% by weight is so poor that it cannot be subjected to cold rolling, or even to warm rolling.
Regarding the technique directed to the industrial-scale production of steel sheets having an Si content of around 6.5% by weight, hot rolling at a low temperature and under a high reduction, is disclosed in Japanese Patent Application Laid-Open (JP-A) Sho-61-166923, and a method is disclosed for processing steel for Si diffusion penetration, in JP-A Sho-62-227078.
However, the technique disclosed in JP-A Sho-61-166923 requires delicate control of the rolled steel texture for seemingly reducing the brittleness of the steel. Therefore, in the disclosed method, the steel must be strictly controlled in production, and it is difficult to stably produce steel sheets on an industrial scale according to the method. On the other hand, the technique disclosed in JP-A Sho-62-227078 requires specific diffusion coating with Si, and is therefore extremely disadvantageous for industrial production of steel sheets, as being too expensive.
An increase of the Si content in steel up to 6.5% by weight can bring about an increase of specific resistivity to only the level of at most 80 .mu..OMEGA..multidot.cm or so. In particular, for steel sheets having an Si content not larger than 3.5% by weight, that could be produced in ordinary industrial rolling methods, the sheets could have a specific resistivity of up to the level of 50 .mu..OMEGA..multidot.cm or so. In other words, a further increase of the electric resistance of steel to be attained by Si addition only is limited, and the mere addition of Si to steel is insufficient for obtaining steel having good high-frequency magnetic properties.
In addition, Si steel is said to be further problematic in use for iron cores, as having poor corrosion resistance.
On the other hand, it is known that Al is effective for increasing electric resistance of steel, like Si. Al does not so greatly reduce the workability of steel. Therefore, substituting for a part of Si in steel with Al would seem to be effective for improving the workability of Si steel while increasing its electric resistance. For example, steel containing 3% by weight of Si and 0.7% by weight of Al has better workability than Al-free steel containing 3.7% by weight of Si. Yet both have nearly the same magnetic properties. However, such Al-containing steel is disadvantageous in that Al is more expensive than Si, and that Al causes significant reduction of magnetic flux density of the Al-containing steel. For another type of Al-containing steel having an Si content of not smaller than 3% by weight, in which the total of Si and Al is not smaller than 4% by weight, its workability is also poor, and cold rolling of the steel is impossible. For still another type of Al-containing steel in which the total of Si and Al is more than 6% by weight, its workability is so poor that even warm rolling of the steel is difficult. In short, steel sheets containing Si and Al to such a degree that the total of Si and Al therein is less than 4% by weight could be produced on an industrial scale, but without practical benefit because their specific resistivity could not be over 60 .mu..OMEGA..multidot.cm.
Even if the amounts of Si and Al added to steel are increased enough to reduce the iron loss in the resulting Si--Al steel within a high frequency range, the essential workability of the steel would not be improved, the corrosion resistance of the steel would be poor, and that the production costs for the steel would be high.
For improving the corrosion resistance of Si steel, a method is disclosed comprising adding a predetermined amount of Cr to the steel (JP-A Sho-52-24117 and JP-A Sho-61-27352). As in those references, addition of Cr to Si steel is known. However, the magnetic properties of the steel disclosed in those publications are still the same as those of ordinary Cr-free Si steel. The magnetic properties of the steel are not improved to a significant degree by the addition of Cr.